In optical metrology, interferometers are used for the measurement of the topography of test piece surfaces, and also of the wavefront quality of optical systems or material samples in transmission. The interferometers are typically, but not exclusively, of the Fizeau type, Twyman-Green type, or Mach-Zehnder type or variants thereof. They consist in general of a light source which provides a beam of light, some means to expand the beam to the required measurement diameter, a beam splitter to split the beam into a test beam and a reference beam, a means to recombine the test and reference beams, and, finally, an optical system to relay both beams to a detector sensing the interferogram. Depending upon the measurement requirements at hand, the suitable interferometer configuration is chosen.
The interferogram contains the information about the piece under test encoded in the phase of the interference fringes. The interferogram can be displayed for visual observation, either directly on a screen or on a video monitor. The interferogram also can be numerically analyzed in a computer. For such a numerical analysis, the detector typically consists of a video camera with a digitized output signal. Special phase detection algorithms exist which allow determining the phase of the interferogram with a very high degree of accuracy. Such algorithms are described in the article by Klaus Freischlad and Chris Koliopoulos entitled Fourier Description of Digital Phase-Measuring Interferometry, J. Opt. Soc. AM., Volume 7, Pages 542-551, April, 1990. These algorithms necessitate that the fringe phase is changed during the data acquisition, most often implemented by moving the reference optic with a PZT actuator.
Typically, lasers are used as the light sources for the interferometer. Because of the narrow spectral bandwidth, with the corresponding high degree of temporal coherence of the lasers used, the optical path difference between the test beam and the reference beam of the interferometer does not need to be matched; and the usage of the interferometer becomes simple. Typical laser sources, however, also have a high degree of spatial coherence, that is they essentially act as a point source for the interferometer. This high degree of spatial coherence again simplifies the interferometer usage, but also results in a loss of measurement accuracy due to coherent noise.
The coherent noise results from light scattered at microscopic imperfections and contaminations of the interferometer optics as the interferometer beam passes from the light source through the illuminating optics to the test piece and reference optics, and subsequently, through the imaging optics of the interferometer to the detector. At the detector of the interferometer, where the interference fringe pattern (interferogram) is formed, there are not only the unscattered reference beam and test beam, but also one or more spurious scattered beams. With a point source, these spurious beams create a so-called speckle pattern on the detector. Also, because of the point-like nature of the light source, the spurious beams change the phase distribution of the detected interferogram, from which the topography of the surface or wavefront under test is to be determined. To a fine degree, a smooth test surface thus does not lead to a smooth measurement, but rather to a rough and noisy topography measurement. This erroneous measurement component is called coherent noise.
The light scattering at the optical surfaces of the interferometer cannot be completely avoided, since in practical instruments all optical surfaces have a finite surface roughness. Also, dust and other contaminations contribute to light scattering. Consequently, in practice, the measurement accuracy of a surface is limited to about five to ten nanometers (nm) for high quality interferometers using fully coherent laser light from the source to the detector.
Various techniques have been employed to improve the measurement accuracy of interferometers by reducing coherent noise. Less susceptibility to coherent noise also allows for less stringent requirements for the surface quality of the interferometer optics. Different ones of the prior art techniques for providing this improvement of measurement accuracy are described in the following paragraphs.
The U.S. Pat. No. 4,201,473 to Domenicali discloses an interferometer using a laser point source which employs a rotating ground glass in an intermediate image plane of the interferogram. The intermediate image then is relayed to the detector. The rotating ground glass acts, to a large extent, as a spatially incoherent object for the subsequent imaging relay. As a result, the contribution to the coherent noise by the imaging relay is much reduced. The illumination optics and imaging optics, however, before the ground glass, still create coherent noise which appears in the interferogram. In addition, the rotating ground glass also can lead to streak-like artifacts in the measurement. One way of reducing the residual coherent noise from the illuminating optical system, as well as the artifacts from the rotating ground glass, consists of de-focusing the image of the ground glass onto the detector. This, however, also causes the image of the test-piece to be blurred; and fine test piece detail at high spatial frequencies is lost.
In the U.S. Pat. No. 5,357,341 to Kuchel the angle of the illuminating light from the interferometer is varied while measurements are acquired and averaged. In the device of the Kuchel patent, because of the different angles of illumination, the coherent noise pattern in each individual map is superposed at different positions on the surface or wavefront map; and the averaging process leads to a reduction of the coherent noise at high spatial frequencies. Since the test piece is imaged onto the detector, its image position is fixed; and the averaging does not lead to a loss of resolution of fine detail on the surface or wavefront under test. Alternatively, the test object is moved with respect to the interferometer optics; and individual measurements at different test piece positions are averaged. For the average, the individual surface or wavefront maps are superposed in such a way that the test piece motion is eliminated. Thus, the coherent noise is displaced in each map while the test piece is stationary. In the average of the individual maps, the coherent noise is reduced while the test piece topography is obtained without loss of resolution. A disadvantage of this technique, however, is that it requires the averaging of a very large number of individual maps. This often is not feasible because of the long data acquisition times required to do this.
In another technique, a low pass filtering data processing step in a computer is applied to the measured surfaces or wavefront topography map. Thus, the coherent noise at high spatial frequencies is reduced. The high spatial frequency content of the actual surface or wavefront under test, however, also is reduced; and fine surface or wavefront detail is suppressed.
In the U.S. Pat. No. 5,737,081, to Freischlad a device is disclosed in which an extended spatially incoherent source is used in a non-telecentric equal path Mach-Zehnder configuration. The large source extent makes any scattered light incoherent to the test and reference beams. As a result, no speckle pattern is present at the detector; and measurements with exceptionally low coherent noise can be made. The non-telecentric Mach-Zehnder configuration with the extended source, however, is suitable only for testing flat surfaces. Consequently, the device of this patent is not a general purpose interferometer configuration for optical testing.
In the devices disclosed in the U.S. Pat. Nos. 4,732,483; 4,869,593 and 4,948,253, to Biegen a technique is used in which a laser beam illuminates a rotating ground glass to cause an extended spot to appear on the ground glass surface. This extended spot on the rotating ground glass then constitutes the effective light source for the interferometer. Because of the extended source, the spurious scattered light has a low degree of coherence with the test and reference beams. Consequently, the speckles due to the spurious scattered light are very much reduced; and also, the coherent noise on the measured surface or wavefront topography is reduced. The combination of the laser with the rotating ground glass allows for an extended monochromatic source with a high degree of temporal coherence and a low degree of spatial coherence. Such a light source reduces the coherent noise while not requiring an equal path interferometer configuration with matched optical paths for the test beam and reference beam. This device can be employed in a variety of interferometer configurations for general optical testing. A disadvantage, however, is in the appearance of streak-like artifacts from the rotating ground glass which are still superposed on the topography measurement due to the limitation of rotation speed of the ground glass. In addition, the rotating ground glass in the interferometer often leads to vibrations of the test set up, resulting in reduced measurement accuracy. This is true even if the motor for rotating the ground glass disk is a precision motor, since any vibrations produced by the motor and the rotating disk are transferred to the frame or base on which the interferometer light source and motor are mounted.
It is desirable to provide an interferometer system for optical testing which has high spatial resolution and improved measurement accuracy, especially at higher spatial frequencies, and which overcomes the disadvantages of the prior art devices mentioned above.